The Two Hybrid System (THS) is a method that uses transcriptional activity as a system to detect protein-protein interactions. A gene fusion is constructed to encode the DNA-binding domain of the yeast transcription factor GAL4 as a hybrid with any protein ‘X’ (usually a defined mammalian protein being the “bait” binding target). An additional gene fusion construct will encode the transcription activation domain of GAL4 fused to any protein ‘Y’ (usually a library of diverse proteins, the “fish”) (Fields et al., 1994). Whenever an X-Y interaction does occur, it will bring the activation domain close to sites on the DNA recognized by the GAL4 DNA-binding domain, therefore resulting in the expression of a flanking reporter gene regulated by these DNA sites. The reporter genes commonly used include: 1) lacZ, which produces blue colonies on plates or filters containing X-Gal; and 2) His3, a yeast gene involved in histidine biosynthesis, required for growth of host yeast cells.
Recently, Fields and his team have used the THS for screening a library of random peptides, instead of a cDNA library, in order to find peptides capable of binding to the retinoblastoma protein (Rb) (Yang et al., 1995).
The receptor system for Interleukin-6 (IL-6) is composed of two distinct receptor “subunits” designated gp80 and gp130 (see Hirano et al., 1994).
The IL-6 type cytokines elicit their signal through receptors that share the gp130 protein. Upon ligand binding gp130 homo- or heterodimerizes with the LIF and OSM receptor, thereby activating associated JAK tyrosine kinases. The JAKs phosphorylate the signal transducer (gp130) and latent transcription factors of the STAT family (Signal Transducer and Activator of Transcription), like STAT1 and STAT3 in the case of IL-6. STAT factors dimerize, translocate to the nucleus and bind to enhancer elements of IL-6 responsive genes (Lüttiken et al., 1993).
Deletion analysis of the intracellular domain of gp130 has defined short stretches of amino acids known as box1 and box2 sufficient to impart both mitogenic activity and binding of JAK proteins (Vanderkuur et al., 1994): these activities were also observed when the binding sites of STATs were deleted. Therefore two functions can be ascribed to JAK kinases: 1) activation of STAT-mediated gene expression; 2) activation of STAT-independent mitogenic activity at least in some hematopoietic cells.
Additional kinases are known to associate with to the intracellular portion of gp130, such as Hck, Fes, Btk and Tec (Matsuda et al., 1995). However these interactions have not been elucidated at the molecular level. Moreover, Tanner et al. have demonstrated that the box1 domain of cytokine receptors is required but not sufficient for interaction with JAK kinases and have suggested that the box1 sequences cooperate with other cytoplasmic domain sequences to effect JAK kinase association (Tanner et al., 1995). Even the molecular counterpart on JAK kinases of box1 and box2 has not been defined.
Synthetic peptides that inhibit IL-6 activity have been described in the International Patent Application WO 97/13781 (YEDA), which relates to peptides derived from the gp80 protein.